The present invention relates to a method of improving the reproducibility of non-invasive measurements of concentration of constituents in arterial blood, primarily spectroscopic and Kromoscopic measurements. In particular, the invention uses a mechanical method, elevation of the measurement site above the heart, to provide greater reproducibility of results.
One of the major problems with spectroscopic and other non-invasive measurements is the reproducibility of results. While measurements may be made which give results that can be correlated with invasive measurements, there are often problems reproducing the run-to-run results and even inconsistency among the data within a run. Although the use of methods such as Kromoscopy, the basics of which are described in the U.S. Pat. No. 5,321,265 entitled NON-INVASIVE TESTING, can provide sufficiently sensitive measurements in order to provide blood glucose or hemoglobin concentration, even these improved measurements may be plagued by the problems of reproducibility. While some of the other improvements described in the related Kromoscopy patents, such as the use of detectors utilizing congruent sampling (see U.S. Pat. No. 5,434,412 entitled IMPROVEMENTS IN NON-SPECTROPHOTOMETRIC MEASUREMENT OF ANALYTE CONCENTRATIONS AND OPTICAL PROPERTIES OF OBJECTS) or coded illumination (see U.S. Pat. No. 5,424,545 entitled NON-INVASIVE NON-SPECTROPHOTOMETRIC INFRARED MEASUREMENT OF BLOOD ANALYTE CONCENTRATIONS) improve accuracy and reproducibility, any variation in the viewed sample can lead to reproducibility problems.
The use of pulsatile measurements (see U.S. Pat. No. 5,434,412 entitled IMPROVEMENTS IN NON-SPECTROPHOTOMETRIC MEASUREMENT OF ANALYTE CONCENTRATIONS AND OPTICAL PROPERTIES OF OBJECTS) is based, in part, on the hypothesis that the change in blood volume caused by the arterial pulse provides the only variation against a constant optical background. While this is a fairly good approximation, in reality there are variations in venous pressure and volume produced by a variety of physiological variables which means that the background is not really constant. This is particularly prevalent at the measuring site of choice for most non-invasive measurements, the finger. The flow of blood through the finger involves a complex network of arteries, veins, capillaries, arteriovenous anastomoses, venules, and arterioles. In particular, blood flow is partially shunted through the anastomoses, primarily to control heat loss through the extremities, at a rate which varies depending on blood pressure and temperature. For example, as the finger is chilled, the flow through the shunt vessels decreases.
Non-invasive measurements making use of the pulsatile nature of blood in general cannot distinguish between pulsatile flow in the arterial circulation and time-variant changes in flow, e.g., pulsatile flow, in the venous circulation, as the transit time for the pressure pulse is extremely short compared to the pulse frequency. Therefore, any pulsatile flow in the venous circulation is seen as a part of the overall pulsatile flow. If the constituent or constituents of interest have different concentrations in arterial and venous blood, venous pulsation can cause an error in measurement, and variability in venous pulsation produces a consequent variability in the measurement.
When the finger is elevated above the level of the heart, the larger venous vessels and some of the anastomoses will drain by gravity toward the heart. This reduction in venous volume and in the volume of the shunt vessels thereby reduces the shunting effect and the venous pulse variation. The result of this manipulation is therefore both a stabilization of the optical background and a reduction in the amount of venous pulsation.
For measurements of oxygen saturation in arterial blood via pulse oximetry, the reduction in venous pulsation has been theorized as an explanation for some of the effects found (See Kim et al. xe2x80x9cPulse Oximetry and Circulatory Kinetics Associated with Pulse Volume Amplitude Measured by Photoelectric Plethysmography.xe2x80x9d Anesth. Analg. 65: 1333-9 (1986)). However, this concept has not been tested or even suggested with respect to concentration measurements. The variation found by Kim et al. was quantified using pulse oximetry measurements, which are measurements of ratios of oxygenated to deoxygenated hemoglobin rather than absolute concentration measurements, and were sufficiently small to be within the normal variance of the instrument. Accordingly, there is not even a suggestion in Kim et al. that elevation would provide any benefit in reproducibility.
Accordingly an object of the invention is to provide a method of providing increased reproducibility of results in non-invasive measurements of concentration.
Another object of the invention is to provide a method of improving accuracy of results of a non-invasive measurement.
These and other objects and features of the invention will be apparent from the following description and the drawing.
The present invention features a method of improving the reproducibility of non-invasive measurements for concentration of a variety of substances in blood. The method is based, in part, on the recognition that reducing the variability of the venous volume and the effects of venous pulsations can lead to an improvement in the overall variance of measurements when an arterial pulsatile measurement is utilized.
The method includes the steps of placing the portion of the body in which a concentration measurement is to be made in a position whereby it is elevated above the heart during the measurement cycle, taking an optical measurement of the body portion while elevated, and calculating concentration from the optical measurement. While any type of optical measurement of the cardiac pulse such as spectroscopic measurements can be utilized and will be improved by this method, Kromoscopic type measurements are preferred. The preferred optical ranges are near infrared measurements and the preferred body portion is a finger. The measurements should be made such that multiple measurements may be made on the same arterial pulse, thereby minimizing the variance.
The preferred optical system for use with the method of the invention utilizes broad band illumination and detection, with multiple detectors or multiple illumination sources having coded detection thereof. In another variant, polarized light is used as the illumination source and changes in the polarization or amount of polarized light having the same polarization are detected. Preferably, congruent illumination and/or congruent sampling are used and a restricted solid angle is used for measurement. Details of preferred Kromoscopic and spectroscopic apparatus, methods and methods of minimizing variance, other than the elevation aspect, are found in U.S. Pat. Nos. 5,321,265; 5,424,545; 5,434,412; 5,818,044; 5,818,048; 5,672,875; and U.S. patent application Ser. No. 09/073,574. Disclosures of all of the aforementioned patents and patent application are hereby incorporated by reference.
For purposes of the present invention, the following terms are used:
The term xe2x80x9copticalxe2x80x9d includes not just use of visible light but also infrared, ultraviolet, or any type of radiation source which is utilized for illumination and/or detection. Optical also includes polarized light, fluorescence, and other types of radiant energy detection and illumination known to those skilled in the art.
The term xe2x80x9celevatingxe2x80x9d means arranging such that the body part is measured is at a physical location above the heart, sufficient to cause drainage of venous blood from the body part. This may be accomplished by raising the hand above the heart with the patient vertically or horizontally positioned so long as the heart itself is sufficiently lower than the body portion being measured.
The terms not specifically defined herein are given their ordinary meaning except to the extent they are defined differently in one of the patents which is incorporated herein by reference, or if there is a different usage common in the industry.